1. Field of the Invention
The present invention relates to a structure of a pneumatic tire (hereinafter referred to as a “tire”).
2. Description of the Related Art
A portion to come in contact with the road surface of a tire is particularly referred to as a tread. The tread has a tread rubber layer. The physical properties of a rubber constituting the tread rubber layer directly influence the performance of the tire. Conventionally, importance has been particularly attached to a wet grip performance and a rolling resistance in the performance of the tire. In the case in which the wet grip performance is enhanced, however, the rolling resistance generally tends to be increased. For this reason, the tire has conventionally been improved to suppress a rise in the rolling resistance with an enhancement in the wet grip performance. More specifically, silica is blended with the rubber constituting the tread rubber layer in place of carbon black to be a reinforcing agent. Alternatively, the tread rubber layer is constituted to have a two-layer structure including outer and inner layers, and the outer layer is constituted by a rubber having an excellent grip performance and the inner layer is constituted by a rubber of a low heat generating type. This has been disclosed in Japanese Laid-Open Patent Publication No. 2003-104009, for example.
In some cases in which the silica is blended with the rubber constituting the tread rubber layer, however, the silica has a poor conductiveness and static electricity is therefore stored in a vehicle to which the tire is attached. In order to prevent the storage, polyethylene glycol ester or the like is blended as an antistatic agent with the rubber constituting the tread rubber layer in some cases. Even if such an antistatic agent is used, however, the following trouble is made. More specifically, when the amount of the polyethylene glycol ester to be blended is small, an antistatic effect is reduced. On the other hand, when the amount of the polyethylene glycol ester to be blended is large, there is a problem in that the antistatic effect is obtained but the abrasion resistance performance of a tire is deteriorated by blending the antistatic agent.
In order to eliminate the drawback, conventionally, a conductive layer is formed and a conductive member is provided on an inside in the radial direction of the tread rubber layer. The conductive member is connected to the conductive layer and is provided to penetrate through the outer peripheral surface (tread surface) of a tread portion. This has been disclosed in Japanese Laid-Open Patent Publication No. 9-71112, for example. When the conductive member is thus disposed, static electricity is grounded through the conductive member. Consequently, the antistatic effect can be obtained. However, the conductive member is provided in the radial direction of the tire. Therefore, there is a problem in that a two-layer structure to reconcile a high wet grip performance to a low rolling resistance is formed well with difficulty.